1. Field of the Invention
The present invention relates to a DC power supply apparatus and more particularly, to a DC power supply apparatus that reduces consumption power in a low-load state.
2. Description of the Related Art
In a switching power supply of a flyback system that is a DC power supply apparatus in the related art, a switching operation of a field effect transistor (hereinafter, simply referred to as FET) that functions as a switching element to switch a primary voltage of a flyback transformer is controlled as follows. For example, ON/OFF of the switching FET is controlled based on a voltage corresponding to a secondary output voltage, a voltage corresponding to a current flowing through the FET, and a drain/source voltage of the FET. Specifically, the FET is turned on using a known power control IC, under the condition that the voltage corresponding to the secondary output voltage exceeds a first reference voltage and the drain/source voltage of the FET is a second reference voltage or lower. Meanwhile, the FET is turned off, under the condition that the voltage corresponding to the secondary output voltage becomes lower than the voltage corresponding to the current flowing through the FET.
In this DC power supply apparatus, for example, as disclosed in Japanese Patent Application Laid-Open No. 2000-148265, an ON period of the FET in a low-load state is shortened to reduce the consumption power in the low-load state.
However, in the DC power supply apparatus according to the related art, since a switching frequency (the number of times) of the switching FET is maintained, there is a limitation in the reduction of the consumption power in the low-load state.
The reduction of the consumption power in the low-load state becomes one of serious problems to be resolved in an electronic system that is equipped with the DC power supply apparatus. For example, when the operation of the electronic apparatus that is equipped with the DC power supply apparatus is in a waiting state, that is, when the electronic apparatus is in the low-load state, the consumption power can be reduced. Therefore, power of the electronic apparatus can be saved. That is, a user can further reduce consumption power at the time of using the electronic apparatus, thereby reducing a cost generated when the electronic apparatus is used. For this reason, it has been increasingly required to reduce the consumption power of the DC power supply apparatus in the low-load state.